70 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



(320.) 

 Imperfec- 

 tions of at- 

 mospheric 

 engine. 



(321.) 

 Watt's 

 principle 

 of separate 

 condensa- 

 tion. 



When Watt discovered by means of his model that 

 the condensation and loss of steam in the cylinder at 

 every stroke of the piston exceeded that which is use- 

 fully employed in producing the vacuum, he proceed- 

 ed very methodically to ascertain the chief numerical 

 data or constants upon which the working of steam- 

 engines depend ; as, the bulk of steam of given elas- 

 ticitv compared with that of the water producing it, the 

 elasticities of steam at different temperatures, espe- 

 cially above the boiling point, the evaporating power 

 of a pound of coals, and finally, the expenditure of 

 steam and of injection-water for a single stroke. The 

 last, the amount of cold water required effectually to 

 condense the cylinderful of steam, appeared so very 

 large that it at once betrayed the source of the waste- 

 ful expenditure of fuel. All the heat which was ab- 

 stracted by the injection-water had to be supplied 

 afresh at a vast expense of steam, and consequently 

 of coal, before a fresh stroke could be made. The 

 mere amount of steam necessary to fill the cylinder 

 and elevate the piston was a trifle in comparison. He 

 naturally ascribed it to the absorption of heat by the 

 material of the cylinder, which no doubt was an im- 

 portant element ; and he tried cylinders of different 

 materials, and ascertained (in most instances for the 

 first time) the specific heats of the substances used. 

 But still the amount of injection-water remained un- 

 accounted for, and the regulation of it was the great 

 difficulty of the old engine. If a tolerable vacuum 

 was desired, the cylinder must be flooded with cold 

 water, and all the heat abstracted must be restored 

 before the next stroke was produced. If, on the other 

 hand, the injection was imperfect, so that the con- 

 densed steam and water together retained a high 

 temperature, an atmosphere of vapour so elastic 

 spoiled the vacuum, and in many cases the working 

 efficiency of the engine was reduced to about half of 

 the atmosphei'ic pressure. 



The theory of latent heat, showing that steam con- 

 tains as much heat as if the water yielding it had 

 been raised through 1000 of temperature, gave a 

 clear explanation of the enormous amount of injec- 

 tion-water necessary. How to escape from the diffi- 

 culty was another question, which was solved solely 

 by Mr Watt, who, early in 1765, first conceived the 

 idea of retaining the cylinder always at the boiling 

 heat, and effecting condensation in a separate vessel 

 kept constantly as cold as possible, in which injection 

 should also take place, and into which the steam of 

 the cylinder would spontaneously rush, on a commu- 

 nication being opened at the proper instant. The suc- 

 cess of this device was complete, and it was speedily 

 followed by a series of admirable and mutually de- 



pendent improvements which Watt introduced. To 

 keep the cylinder as warm as possible, it was first 

 provided with a cover, through which the piston-rod 

 passed by means of a stuffing-box. Then steam it- 

 self was employed to press down the piston, instead 

 of atmospheric air which cooled the cylinder injuri- 

 ously. 



Having thus steam on each side of the piston, the (322.) 

 step was easy to make the engine a double-acting Double-act- 

 one ; so that the volume of steam which by its elas- in 

 ticity impelled the piston during its descent, being 

 thrown into the condenser, produced the vacuum 

 necessary to make the ascending stroke equally ad- 

 vantageous. 1 But in pumping-engines, which had 

 hitherto been the main employment of steam, or fire 

 engines (as they were then called), it was sufficient 

 that the power was produced in one direction, that 

 is, to lift the pump-rods ; 2 but the double action of a 

 pushing and pulling force was evidently applicable 

 to every sort of machinery by the use of a crank and 

 fly-wheel. A difficulty, however, was experienced in 

 applying this push-and-pull movement to the oscil- 

 lating beam which had so long formed a part of the 

 atmospheric engine, and which it wag so convenient to 

 use. Mr Watt obviated it with his customary saga- Parallel 

 city, and succeeded in inventing a connection of rigid motion, 

 jointed links which effectually united the piston-rod 

 with the end of the beam, the former moving in a 

 vertical straight line, the latter in a circular arc, so 

 that pressure was transmitted in the manner of either 

 push or pull without the slightest practical incon- 

 venience. No doubt the locus of the motion of the 

 piston-rod head is not rigorously straight. It is, when 

 fully developed, a curve of the fourth order, resem- 

 bling a very elongated figure 8 ; but the portion of 

 the curve in use is practically straight. This happy 

 solution, the most elegant perhaps in the science of 

 pure machinery, or kinematics, was not the result of 

 a formal mathematical research, but presented itself 

 to the mind of Watt, as he tells us, almost intui- 

 tively. It became from henceforth an important ele- 

 ment of mechanical combinations. 



Other methods of producing direct rotation oc- (323.) 

 curred to the ingenious mind of Watt, and many of Expansive 

 them were shown to be practicable by being actually w 

 executed in model size. But the next great practi- 

 cal improvement introduced by him was the princi- 

 ciple of expansive working, whereby a saving of steam 

 is produced (particularly in single-acting engines),, 

 second in importance only to the result of the sepa- 

 rate condensation. This consists in allowing the 

 steam to flow in upon the piston during only the first 

 part of the stroke ; the rest being completed partly 



1 This happy idea seems to have been suggested by Dr Small of Birmingham in a letter to Watt, of date 5th November 1769. 



" I see no reason why you should not condense at both ends of your cylinder, and drive all before you, back stroke and 



fore stroke." Mechanical Inventions of James Watt, i. p. 81. The uncommon sagacity of Dr Small, and the important services 

 which he rendered in many ways to Watt, come out clearly for the first time in this work. 



2 So associated were these engines with the operation of raising water, that we find they were at one time used to supply 

 common water-wheels with a constant stream, and thus to produce continued motion for mill-work, &c. 



